【作者】 张楠；

【导师】 陈福义；

【作者基本信息】 西北工业大学 ， 材料学， 2019， 博士

【摘要】 纳米合金在减少贵金属用量的同时可以增强催化活性,在催化领域引起了广泛关注。但是,由于实验技术的缺乏,目前人们对合金的成分-结构-性能之间的关系无法仅通过实验研究来解决。因此,从原子-分子尺度研究合金的结构和电催化反应过程尤为重要。与此同时,随着现代计算机技术的长足进步,量子计算开始越来越多的应用于电催化反应的研究中。研究表明,Ag在碱性环境下稳定性高,Ag基纳米合金催化剂在碱性燃料电池中极具发展潜力。但纳米合金的高催化活性起因还未达成统一的认识,其用于催化反应的机理也不完全清楚。另外,纳米颗粒的多样性导致了难以获得低能量结构,合金团簇的微观结构也存在争议。因此,针对这些问题,本文开展了相关的研究工作,研究结果可为纳米合金催化剂的设计提供基础理论支持。本文采用遗传算法全局优化方法和密度泛函理论（DFT）研究了38原子AgCu团簇的结构。结果表明,多二十面体（p Ih）的Ag₃₂Cu₆核壳团簇原子模型比截角八面体（TO）的更稳定。新发现的p Ih-Ag₃₂Cu₆核壳团簇在氧还原反应（ORR）中具有潜在的应用前景。进一步研究O₂在p Ih-Ag₃₂Cu₆核壳团簇上的吸附与解离,观察到在最佳吸附桥位b2上,O₂解离的活化能是0.715 e V,d带中心是-3.395 e V,态密度在费米能级处最大,表明O₂与p Ih-Ag₃₂Cu₆核壳团簇之间的最多电荷转移影响了催化活性。本工作修正了先前关于Ag₃₂Cu₆核壳纳米颗粒不适合作为ORR催化剂的观点,证实了AgCu纳米合金是替代碱性燃料电池中贵金属Pt基催化剂的潜在候选材料。针对AgCu纳米合金,本文通过理论和实验方法研究了核壳和合金AgCu体系ORR的催化性能。计算了纯Ag,核壳Ag₃Cu@Ag和合金Ag₃Cu的活化能分别为0.532、0.443和1.137 e V,表明核壳Ag₃Cu@Ag的ORR活性比纯Ag和合金Ag₃Cu高。计算了纯Ag,核壳Ag₃Cu@Ag、和合金Ag₃Cu的工作电位分别为0.737、0.761和0.675 V,表明核壳Ag₃Cu@Ag具有最高的工作电位和最低的过电位0.469 V。采用脉冲激光沉积法（PLD）制备了AgCu双金属催化剂,其中核壳AgCu@Ag催化剂的ORR活性大于合金Ag₃Cu催化剂,这与DFT计算结果一致,说明,构造核壳结构是一种提高AgCu双金属纳米合金ORR活性的有效方式。为了寻找Ag基ORR催化剂合适的掺杂元素,本文研究了3d过渡金属M掺杂Ag（111）表面的稀合金M₁Ag（111）的ORR催化性能,并将AgCu纳米合金进一步优化为三元Cu Mn Ag纳米合金提高了ORR催化性能。通过对表面偏析和混合能的研究发现,当M处于亚表面（2L）时,M₁Ag（111）稀合金的稳定性高于M在Ag（111）面的表面（1L）。Mulliken电荷分析表明,这种亚表面稳定性是由表面银原子的相对正电荷转移引起的。进一步研究了2L-M₁Ag（111）催化剂的电子结构,发现2L-Cu₁Ag（111）,2L-Ni₁Ag（111）和2L-Zn₁Ag（111）是具有自由原子d态的稀纳米合金。通过计算ORR自由能,发现2L-Mn₁Ag（111）和2L-Cu₁Ag（111）具有较低的过电位为0.459和0.468 V,在此基础上预测三元2L-Cu₁Mn₁Ag（111）的过电位为0.450 V。根据预测采用PLD制备出三元Cu Mn Ag纳米合金,电化学实验结果表明三元Cu Mn Ag催化剂的过电位为0.50 V接近商业Pt/C催化剂。为了探索Ag基合金在阳极催化剂方面的应用,本文通过DFT研究了Pd掺杂Ag稀纳米合金的甲酸盐氧化反应（FOR）催化性能。计算了FOR的自由能和活化能,结果表明与Pd₂Ag（111）和Pd₃Ag（111）稀合金相比,Pd₁Ag（111）单原子合金（SAA）的催化活性最高,其限速步骤的最低热力学限制能在直接联合路径为0.026 e V,在直接解离路径为0.084 e V,且直接解离路径存在最低活化能为0.774 e V。分析了Pd Ag稀纳米合金的电子结构,观察到Pd₁Ag（111）SAA的分态密度图中从-0.75到-2.0 e V呈现极窄的尖峰,这是由于单个Pd原子的自由原子状电子结构所致。比较了孤立的单原子构型和聚合的团簇构型的稳定性,发现与Ag（111）表面上聚集的Pd₂和Pd₃原子团簇相比,分离的Pd单原子的稳定性有所提升（分别为-0.041 e V和-0.097 e V）,这证实了Pd₁Ag（111）SAA的潜在应用。总的来说,本文进一步阐明了FOR机理的理论概况,并为设计原子水平的催化剂提供了一种新的策略。为了进一步研究表面结构对Ag催化剂ORR活性的影响,本文通过理论和实验方法研究了Ag（hkl）表面的ORR催化性能。采用过渡态搜索法分析了Ag（hkl）表面的ORR三种可能机制的反应路径,表明相比于平面Ag（111）和Ag（200）表面,台阶Ag（220）表面的O₂质子化的具有最低活化能为0.504 e V。计算了Ag（hkl）表面的ORR自由能,表明台阶Ag（220）表面的ORR过电位为0.457 V,接近Pt（111）催化剂的过电位0.441 V。说明台阶Ag（220）表面是Ag（hkl）表面中ORR活性最高的。采用旋转圆盘电极法测量了Ag（hkl）表面的ORR极化曲线,结果表明催化活性顺序为Ag（220）>Ag（111）>Ag（200）。实验和计算结果均表明,对于具有确定形貌的纯银材料,台阶Ag（220）表面是高ORR活性的来源之一。这一新的认识为了解单金属银的ORR反应机理,设计基于纯银的先进催化材料提供了基础。更多还原

【Abstract】 Nanoalloys have an advantage of enhancing catalytic activity while reducing the loading of noble metals,which have attracted extensive attention in the field of catalysis.However,due to the lack of experimental technology,the relation of alloys composition,structure,and properties can be hardly solved through experimental research.Therefore,it is particularly important to study the alloys structure and their electrocatalytic reaction process on an atomic-molecular scale.At the same time,with the rapid development of modern computer technology,quantum computing has been more and more popularly applied in the study of electrochemical catalysis.According to previous research works,Ag has high stability in an alkaline environment,and Ag-based nanoalloy catalysts have great potential for the development of alkaline fuel cells.However,the principle of high catalytic activity of nanoalloys has not been adequately understood,and the mechanism of their application in catalytic reaction has not been fully clarified.In addition,the diversity of nanoparticles makes it difficult to obtain a low-energy structure,and the microstructure of alloy clusters is also controversial.Therefore,in view of these problems,carrying out further relevant research work would contribute to providing basic theoretical support for the design of alloy catalysts.In this study,the structure of 38 atoms AgCu cluster was studied by a genetic algorithm global optimization technique and density functional theory（DFT）calculations.The results show that the polyicosahedral（p Ih）Ag₃₂Cu₆ core-shell cluster is more stable than the truncated octahedral（TO）Ag₃₂Cu₆ core-shell cluster of the atomistic models.Therefore,the newfound p Ih-Ag₃₂Cu₆core-shell cluster would have a potential application for an oxygen reduction reaction（ORR）.For the O₂ dissociation on p Ih-Ag₃₂Cu₆core-shell cluster,the density of states at the Fermi energy level is maximal at the favorable absorption site b2,the activation energy barrier for the O₂ dissociation is 0.715 e V,and the d-band center is-3.395e V.This indicates that the catalytic activity is attributed to a maximal charge transfer between an oxygen molecule and the p Ih-Ag₃₂Cu₆core-shell cluster.This work revises the earlier viewpoint that Ag₃₂Cu₆core-shell nanoparticles are not suitable as ORR catalysts and demonstrated that AgCu nanoalloy is a potential candidate to substitute noble Pt-based catalysts in the alkaline fuel cells.As for AgCu nanoalloy,the catalytic properties of ORR in core-shell and AgCu alloys for the oxygen reduction was investigated utilizing both theoretical and experimental methods.The activation energies on the pure Ag,core-shell Ag₃Cu@Ag and alloy Ag₃Cu are 0.532,0.443 and 1.137 e V,respectively,indicating that the ORR activity of core-shell Ag₃Cu@Ag is higher than that of pure Ag and alloy Ag₃Cu.The predicted working potentials of pure Ag,core-shell Ag₃Cu@Ag and alloy Ag₃Cu are 0.737,0.761 and 0.675 V,respectively,indicating that the core-shell Ag₃Cu@Ag nanoparticles have the highest working potential but the lowest overpotential 0.469 V.AgCu bimetallic catalysts were prepared by the pulse laser deposition（PLD）.The core-shell Ag₃Cu@Ag catalysts showed greater ORR activity than the alloy Ag₃Cu catalysts,which is consistent with the DFT calculations.The results indicate that a core-shell atom order should be designed for AgCu bimetallic nanoparticles to enhance their ORR activity.To search for suitable doping elements for Ag-based ORR catalysts,the ORR catalytic properties of dilute alloy M₁Ag（111）doped with M（M=3d transition metal TM）on Ag（111）surface were studied.Furthermore,the catalytic performance of ORR was improved by optimizing AgCu nanoalloy to ternary Cu Mn Ag nanoalloy.Through an investigation into surface segregation and mixing energy,it is unexpected to find that the M₁Ag（111）dilute alloys exhibit higher stability when M is in the subsurface（2L）than that on the surface（1L）of Ag（111）facet.The atomic charge analysis reveals that this subsurface stability is attributed to the relative positive charge transfer of surface Ag atoms.The 2L-M₁Ag（111）catalysts were further investigated for their electronic structure and ORR activities,particularly,2L-Cu₁Ag（111）,2L-Ni₁Ag（111）and 2L-Zn₁Ag（111）are dilute nanoalloys with free-atom-like electronic structure.2L-Mn₁Ag（111）and 2L-Cu₁Ag（111）exhibit an overpotential of 0.459 V and 0.468 V.The ternary 2L-Cu₁Mn₁Ag（111）is theoretically predicted with an overpotential of 0.450 V.Motivated by the prediction,the PLD method is proposed to prepare ternary Cu Mn Ag nanoalloys.Accordingly,these ternary Cu Mn Ag catalysts exhibit an overpotential of 0.50 V,which is close to the predicted overpotential and that of the commercial Pt/C catalyst.To explore the application of Ag-based alloy in anode catalysis,a series of DFT calculations were carried out to investigate the catalytic activity of Pd-doped Ag dilute nanoalloys in formate oxidation reaction（FOR）.Compared with Pd₂Ag（111）and Pd₃Ag（111）dilute alloys,the Pd₁Ag（111）single-atom alloy（SAA）exhibits the highest FOR catalytic activity.The low limiting potential of 0.026 e V for direct association path and a value of0.084 e V for the direct dissociation path,and the lowest activation energy of 0.774 e V for the rate-determining-step in the direct dissociation path.Pd₁Ag（111）SAA exhibits an extremely narrow sharp peak in the partial density of state curves from-0.75 to-2.0 e V,which is due to the free-atom-like electronic structure of the single Pd atom.The isolated Pd single atom is more stable（-0.041 and-0.097 e V,respectively）than the aggregated Pd₂ and Pd₃ atom cluster on the Ag（111）surface.This verifies the potential application of Pd₁Ag（111）SAA in experiments.This work further elucidates the theoretical profile of FOR and provides a new strategy for designing the catalytic reaction at the atomic level.To further study the effect of surface structure on the ORR activity of Ag catalyst,the ORR catalytic performance of Ag（hkl）surfaces was investigated by computational modeling and experimental measurement.The reaction paths in three possible mechanisms of ORR on Ag（hkl）surfaces were analyzed by the transition-state searching method.The stepped Ag（220）surface exhibits an activation energy barrier of O₂ protonation reaction is 0.504 e V,which is the lowest activation energy barrier compared with the flat Ag（111）and Ag（200）surfaces.Furthermore,the theoretical calculations show that the ORR overpotential of stepped Ag（220）surface is 0.457 V,which is comparable to the Pt（111）catalyst with the overpotential of 0.441V.It indicates that the stepped Ag（220）surface is a high ORR active site on the silver surface.The ORR polarization curves of the textured silver surface were measured by the rotating disk electrode method.The experimental ORR activity trend is Ag（220）>Ag（111）>Ag（200）surface for the silver surface.Both the experimental and computational results indicate that the stepped Ag（220）surface is one of high ORR activity origin for pure Ag materials with desirable morphologies.This new insight provides a fundamental understanding of the ORR reaction mechanism of monometallic Ag and is beneficial for the design of advanced catalytic materials based on pure Ag.更多还原